A charged particle beam is circulated and accelerated by an accelerator (circular accelerator) such as a synchrotron, the charged particle beam which is accelerated to be high-energy (mainly, a proton or a carbon ion) is extracted from the circulating orbit, the charged particle beam which becomes a beam-state (will be also referred to as a charged particle beam, or a particle beam) is utilized in a physics experiment or a particle beam therapy such as a cancer treatment in which the charged particle beam is transported by a beam transport system so as to irradiate to an intended object. In a particle beam therapy by an accelerated charged particle beam (hereinafter, will be referred to as a particle beam), a particle beam is transported to an irradiation device which is provided in a particle beam irradiation chamber.
In an irradiation device, a thin beam-like particle beam is scanned and spread by, for example, two sets of deflection electromagnet in directions of two axis which are perpendicular to a beam travelling direction, then, is passed through a scatterer to be spread further, is finally cut out to be a shape of a cancer by a collimator so as to irradiate to a person to be treated. A depth direction irradiation of a particle beam is adjusted to be a depth direction width of a cancer part of a person to be treated by passing the particle beam through a splinter-like filter, for example, so called a ridge filter and spreading a width of energy. The above-mentioned irradiation method of a particle beam is called as a spreading irradiation method. Further, recently, a scanning irradiation method, in which a particle beam whose state is kept thine beam state is scanned only by two sets of deflection electromagnets for beam scanning and is irradiated to a cancer part, is performed.
In a case of a spreading irradiation method, when a particle beam which is accelerated to be high-energy collides with a scatterer or a collimator, finally a body of a person to be treated, radiation such as a neutron beam or a photon beam is generated secondarily. A spot where particle beams which are accelerated to be high-energy are collided and a neutron beam or a photon beam is generated secondarily is called a source of radiation. At this time, a neutron beam which is generated secondarily has energy distribution to the vicinity of energy of incident charged particle beam at most (in a case of particle beam treatment, per nucleon, several hundred MeV). Also, in a case of a scanning irradiation method, frequency of collision with a particle beam and a collimator or a scatterer is low, however, a particle beam is finally irradiated to inside of a body of a person to be treated. Therefore, total amount of a neutron or a photon beam which is generated secondarily is small in comparison with that of a spreading irradiation method, however, high-energy neutron beam or a photon beam is generated.
In a facility where radiation such as a neutron beam or a photon beam is generated, dose limit is specified by laws and regulations. In a particle beam therapy facility, in order to make effective dose of outside a particle beam irradiation chamber to be in a legally permissible range, the intensity of a neutron is attenuated by thickening a thickness of a concrete wall or making a passage from a treatment bed where a person to be treated (patient) is placed in a particle beam irradiation chamber to a door of an entrance of an irradiation chamber to be a labyrinth-like shape (for example, refer to Patent Document 1).
When a particle beam irradiation chamber is designed in shielding manner, two kinds of shielding effects, that is, an effect of bulk shielding and streaming should be considered. Bulk shielding is an effect to attenuate the dose equivalent of a neutron beam or a photon beam which is reached from a source of radiation passing through a concrete wall. In general, when a wall is thick and density of wall material is high, a shielding effect is high. Streaming is an effect of leaking a neutron beam or a photon beam, which is passed through a passage which connects inside and outside of a particle beam irradiation chamber, to the outside chamber. In general, when a passage is longer, cross section of a passage is smaller, and the time of bending is larger, dose equivalent of a neutron beam or a photon beam which leaks outside chamber is reduced by streaming is smaller.
In a case where a shielding design in which a passage is utilized is performed, dose in the vicinity of an entrance of a passage which is a side of a radiation source is maximum dose, and after that dose is attenuated by a distance. Therefore, it is preferable such that dose at the radiation source entrance side of the passage is made to be small as possible. In general, energy of a neutron which is reached without being shielded on the way from a radiation source is not attenuated, therefore contribution to equivalent dose is large. (In strictly speaking, the degree of contribution to dose is different depending on energy, however, in a case of a neutron which is generated in a particle beam therapy, several hundred MeV neutron is generated at most, and contribution to a neutron in a range of several MeV to several hundred MeV is large). Consequently, it is important to attenuate an irradiation dose which intrudes in a passage by reducing the speed of a particle beam with a shielding wall and scattering. Conventionally, not only by making a passage to be a labyrinth-like form but also by forming a convex wall in a labyrinth-like passage, a neutron beam, which is reached from a radiation source to a gateway outside a particle beam irradiation chamber, is attenuated (for example, refer to Patent Document 2).